1. Field of the Invention
The present invention relates to a demodulating apparatus and a demodulating method for demodulating a digital orthogonal frequency division multiplex modulated signal in which an information signal modulates a plurality of carriers whose frequency components are in an orthogonal relationship with one another.
2. Background of the Invention
As a demodulating apparatus for demodulating a digital orthogonal frequency division multiplex modulated signal in which an information signal modulates a plurality of carriers whose frequency components are in an orthogonal relationship with one another, there is proposed a demodulating apparatus for an OFDM (Orthogonal Frequency Division Multiplex) modulated signal (hereinafter referred to merely as an OFDM modulated signal) which is adopted by a DAB (Digital Audio Broadcasting), etc. taking place in Europe.
According to the OFDM modulation, a modulated signal using a multiplicity of carriers whose frequency components are in an orthogonal relationship with one another, encodes data such as audio data or the like, and the encoded data are allocated to each carrier, thereby modulating each carrier, a digital signal in the frequency domain comprised of each modulated carrier is inverse fast Fourier transformed into a digital signal in a time domain, and the digital signal in the time domain is D/A converted. On its demodulating side, by A/D converting such an OFDM modulated signal and then applying the fast Fourier transform to the A/D converted signal, the encoded data allocated to each carrier is obtained.
In the OFDM modulation used by the DAB, when two bits data is allocated with one of the carriers, each carrier is subjected to a QPSK (Quadrature Phase Shift Keying) modulation, so that this modulation is called an OFDM-QPSK.
In the OFDM modulation, the number of points of the fast Fourier transform corresponds to the number of the carriers and differs depending on a mode according to the DAB standard, i.e. for mode 1 it is 1536, for mode 2, 384, for mode 3, 192 and for mode 4, 68. Therefore, in case of mode 1, for example, it will be possible to transmit data of 2 (bits).times.1536=3072 (bits) by the OFDM modulation. This transmissive unit is termed a symbol. In case of modes 1, 2, and 4, a collection of seventy-six symbols is termed a frame. In case of mode 3, a collection of one hundred and fifty-three symbols is termed a frame. Here, a null symbol is not included in the number of symbols in one frame.
Concerning the DAB signal, at present, signals of modes 1, 2, 3 and 4 are known. In the DAB signal, a fundamental period T(=1/2.048 MHz=0.00048828 msec) is defined. Here, a typical DAB signal of mode 1 is shown in FIG. 1. In FIG. 1, both the fundamental period T and a time are indicated. One frame of the DAB signal of mode 1 is 196608 T (=96 msec) and comprised of one null symbol (symbol number 1=0) whose duration is 2656 T (=1.297 msec) and seventy-six subsequent symbols (symbol number 1=1.about.76) whose duration is 2552 T (=1.246 msec).
Each symbol of symbol numbers 1=1-76 is comprised of a guard interval at its starting section whose duration is 504 T=246 .mu.sec) and a subsequent effective symbol whose duration is 2048 T (=1 msec). The effective symbol of each symbol of symbol numbers 1=1-76 contains multicarriers which number k=1536 and whose frequencies are different from each other. A carrier indicated by zero is the one with the central frequency (a period of that carrier is T). A carrier indicated by 1536/2 (=766) is the one with the highest frequency. A carrier indicated by -1536/2 (=-766) is the one with the lowest frequency. Data amount of one symbol is 1536 waves and, its data amount is 1536.times.2 bits, i.e. 48 CU (Capacity unit).times.64 bits.
Whole symbols of symbol numbers 1=1-76 are termed an OFDM symbol.
Taking an example in case of mode 1, the symbol of symbol number 1=0 is termed the null symbol and the symbol of symbol number 1=1 is termed a TFPR (Time Frequency Phase Reference) symbol, respectively. These two symbols are called a synchronization channel (Sync. Channel). The symbols of symbol numbers 1=2.about.4 are called a FIC (Fast Information Channel) and the whole FIC is divided into twelve FIBs (Fast Information Blocks). The remaining symbols of symbol numbers 1=5.about.76 is divided into four so-called CIFs (Common Interleaved Frames).
Incidentally, the duration of each symbol of the DAB signal differs depending on the mode. The duration of each symbol in mode 2 is 1/4 of the duration of each symbol in mode 1. The duration of each symbol in mode 3 is 1/8 of that of each symbol in mode 1. The duration of each symbol in mode 4 is 1/2 of that of each symbol in mode 1.
In other words, the duration of the symbol except the null symbol is 2552 T (=1.246 msec) for mode 1 as described above. However for mode 2, it is 638 T (=2552 T/4) {=312 .mu.sec (=1.246 msec/4)}. For mode 3, it is 319T (=2552 T/8){=156 .mu.sec (=1.246 msec/8)}. For mode 4, it is 1276 T (=2552 T/2){=623 .mu.sec (=1.246 msec/2)}.
Moreover, the duration T/M of the effective symbol within each symbol except the null symbol is 2048 T (=1 msec) for mode 1 as described above. For mode 2, it is 512 T (=2048 T/4) {=250 .mu.sec (=1 msec/4)}. For mode 3, it is 256 T (=2048 T/8) {=125 .mu.sec (=1 msec/8)}. For mode 4, it is 1024 T (=2048/2) {=500 .mu.sec (=1 msec/2)}.
Furthermore, the duration of the guard interval in the symbol except the null symbol is 504 T (=246 .mu.sec) for mode 1. For mode 2, it is 126 T (=504 T/4) {=61.5 .mu.sec (=246 .mu.sec/4)}. For mode 3, it is 63 T (=504 T/8) {=30.75 .mu.sec (=246 .mu.sec/8)}. For mode 4, it is 252 T(=504 T/2) {=123 .mu.sec (=246 .mu.sec/2)}.
A conventional example of a receiving apparatus (demodulating apparatus) for the DAB signal will now be described below with reference to FIG. 2. A DAB signal (shown in FIG. 3A) from a receiving antenna 1 is supplied to a RF (radio frequency) amplifier/frequency converter/IF (intermediate frequency) amplifier 2 where it is RF amplified, frequency converted and IF amplified for obtaining an OFDM modulated signal of baseband, and the OFDM modulated signal is supplied to an A/D converter 3 where it is converted into a time series of digital data.
The time series of digital data from the A/D converter 3 is supplied to a time synchronizing signal generator 7 which generates a time synchronizing signal for every symbol. The time synchronizing signal is supplied to a fast Fourier transform circuit 4 and a data decoder 5 for controlling the fast Fourier transform timing as well as controlling each circuit in the data decoder 5 to be synchronized.
The intermediate frequency signal from the RF amplifier/frequency converter/IF amplifier 2 is supplied to a null detector (envelope detector circuit) 8 which produces a null detecting signal (see FIG. 3B). This null detecting signal is supplied to a frame synchronizing signal generator 9. The frame synchronizing signal generator 9 is a pulse oscillator which generates the frame synchronizing signal. So, it is necessary to make it synchronized with the null detecting signal (FIG. 3B) of the first or second frame of the DAB signal. Thus, by making the frame synchronizing signal generator 9 synchronized with a starting (falling) edge time point of the null detecting signal, the frame synchronizing signal generator 9 will thereafter issue a frame synchronizing signal (see FIG. 3C) corresponding with the starting time point of the null symbol. The frame synchronizing signal is supplied to the fast Fourier transform circuit 4 in which a frame window signal (FIG. 3D) during a frame period except the null symbol is produced.
The time series of digital data from the A/D converter 3 is supplied to the fast Fourier transform circuit 4 where it is converted into a frequency sequence of digital data. The frequency sequence of digital data from the fast Fourier transform circuit 4 is supplied to the data decoder 5 for decoding and decoded data is output to at an output terminal 6. The data decoder 5 is comprised of a frequency deinterleave circuit, a time deinterleave circuit and an error correcting circuit, which are sequentially cascaded.
In this way, because in the conventional demodulating apparatus (DAB receiver) the frame synchronizing signal generator is synchronized with the null detecting signal, when the starting time point of the null detecting signal deviates greatly from the null symbol of DAB signal due to fading or decrease of S/N ratio of the DAB signal, the frame synchronizing signal from the frame synchronizing signal generator also deviates from the starting time point of the null symbol of DAB signal. As a result, the timing of the frame window signal produced in the fast Fourier transform circuit 4 will deviate from the frame period except the null symbol of DAB signal. If an amount of the deviation is large, it will be impossible to estimate the TFPR (Time Frequency Phase Reference) symbol following the null symbol, and besides, it will be necessary to make the frame synchronizing signal generator 9 synchronized over again.